1. Field of the Invention
This invention relates generally to implantable lead assemblies, and, more particularly, to lead assemblies providing improved heat dissipation at an end of the lead having an adjacent electrode. The improved lead reduces or eliminates tissue damage to body structures coupled to the electrode. The lead assembly may be coupled to an implantable medical device (IMD).
2. Description of the Related Art
The human nervous system (HNS) includes the brain and the spinal cord, collectively known as the central nervous system (CNS), and the nerves in the remainder of the body, which collectively form the peripheral nervous system (PNS). The central nervous system comprises nerve fibers that transmit nerve signals to, from, and within the brain and spinal cord. The peripheral nervous system includes nerves that connect the brain to the rest of the body and provide sensory, motor, and other neural signals. The PNS includes the cranial nerves, which connect directly to the brain to control, for example, vision, eye movement, hearing, facial movement, and feeling. The PNS also includes the autonomic nervous system (ANS), which controls such involuntary functions as blood vessel diameter, intestinal movements, and actions of many internal organs. Autonomic functions include blood pressure, body temperature, heartbeat and essentially all the unconscious activities that occur without voluntary control.
Many, but not all, nerve fibers in the brain and the peripheral nerves are sheathed in a covering called myelin. The myelin sheath insulates electrical pulses traveling along the nerves. A nerve bundle may comprise up to 100,000 or more individual nerve fibers of different types, including larger diameter A and B fibers which comprise a myelin sheath and C fibers which have a much smaller diameter and are unmyelinated. Different types of nerve fibers, among other things, comprise different sizes, conduction velocities, stimulation thresholds, and myelination status (i.e., myelinated or unmyelinated).
As used herein, “stimulation” or “stimulation signal” refers to the application of an electrical, mechanical, magnetic, electromagnetic, photonic, audio and/or chemical signal to a neural structure in the patient's body. The signal is an exogenous signal that is distinct from the endogenous electrical, mechanical, and chemical activity (e.g., afferent and/or efferent electrical action potentials) generated by the patient's body and environment. In other words, the stimulation signal (whether electrical, mechanical, magnetic, electro-magnetic, photonic, audio or chemical in nature) applied to the nerve in the present invention is a signal applied from an artificial source, e.g., a neurostimulator.
A “therapeutic signal” refers to a stimulation signal delivered to a patient's body with the intent of treating a disorder by providing a modulating effect to neural tissue. The effect of a stimulation signal on neuronal activity is termed “modulation”; however, for simplicity, the terms “stimulating” and “modulating”, and variants thereof, are sometimes used interchangeably herein. In general, however, the delivery of an exogenous signal itself refers to “stimulation” of the neural structure, while the effects of that signal, if any, on the electrical activity of the neural structure are properly referred to as “modulation.” The effect of delivery of the stimulation signal to the neural tissue may be excitatory or inhibitory and may potentiate acute and/or long-term changes in neuronal activity. For example, the “modulating” effect of the stimulation signal to the neural tissue may comprise one more of the following effects: (a) changes in neural tissue to initiate an action potential (afferent and/or efferent action potentials); (b) inhibition of conduction of action potentials (whether endogenous or exogenously induced) or blocking the conduction of action potentials (hyperpolarizing or collision blocking), (c) affecting changes in neurotransmitter/neuromodulator release or uptake, and (d) changes in neuro-plasticity or neurogenesis of brain tissue.
Thus, electrical neurostimulation or modulation of a neural structure refers to the application of an exogenous electrical signal (as opposed to mechanical, chemical, photonic, or another mode of signal delivery) to the neural structure. Electrical neurostimulation may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. In one embodiment, the electrical neurostimulation involves sensing or detecting a body parameter, with the electrical signal being delivered in response to the sensed body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. In another embodiment, the system may operate without sensing or detecting a body parameter once the patient has been diagnosed with a medical condition that may be treated by neurostimulation. In this case, the system may periodically apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) intermittently throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. The stimulation may be applied by an implantable medical device that is implanted within the patient's body. In another alternative embodiment, the signal may be generated by an external pulse generator outside the patient's body, coupled by an RF or wireless link to an implanted electrode.
Generally, neurostimulation signals to perform neuromodulation are delivered by the implantable device via one or more leads. The leads generally terminate into electrodes, which are affixed onto a tissue in the patient's body. A number of leads may project from an implantable device onto various portions of a patient's body. For example, a number of electrodes may be attached to various points of a nerve or other tissue inside a human body for delivery of neurostimulation.
Occasionally, a patient having an implantable medical device may be subjected to an electrical field, a magnetic field, and/or an electromagnetic field. When an implanted medical system is subjected to one of the aforementioned fields, a coupled signal and/or noise may appear on various portions the implantable medical system, particularly on the leads and electrodes. Depending on the strength of the field, a significant amount of coupled energy may appear on the leads. This coupled energy may cause adverse effects, such as heating of various portions of the implantable system. This heating may damage tissue that is proximate to the portion of the implantable system that experiences the thermal changes.
Turning to FIG. 1, a stylized diagram of a prior art electrode in contact with body tissue is illustrated. The electrode includes a tip end. The electrode may experience an induced current that may flow in the direction indicated in FIG. 1. The induced current (iinduced) may be the result of an electrical field, a magnetic field, or an electromagnetic field applied to the electrode. The induced current may flow through a lead connected at a proximal end to the IMD and at a distal end to the electrode. At the tip of the electrode, the current path is interrupted and the induced current may experience a significant sudden increase in impedance (Imp). The power experienced by the body tissue at the tip end of the electrode is defined by Equation 1.Power=(iinduced)2*Imp  Equation 1The power relating to the current that is induced may be significantly large since it is equal to the square of the induced current [(iinduced)2] multiplied by the high impedance [Imp] at the tip end of the electrode. Therefore, at the intersection of the body tissue and the electrode, a significantly high amount of power may be delivered. Due to the principle of conservation of energy, this power may be converted into and dissipated as another form of energy, such as thermal energy. In other words, at the tip of the electrode a large amount of power is transformed to thermal energy, thereby causing a significant rise in temperature.
Turning now to FIG. 2, a graph relating to an exemplary temperature rise resulting from an induced current is illustrated. After the start-of-scan of a radio frequency (RF) signal, the tip of the electrode associated with the IMD may experience induced current. As described above, this induced current may result in significant thermal energy associated with power flux at the electrode-tissue interface. In some cases, the temperature at the interface may rise asymptotically until a substantial steady-state is reached. For example, as illustrated in FIG. 2, upon the start-of-scan, a significantly rapid rise in the temperature at the end of the electrode may occur. In one example, a temperature rise above 42° C. may be experienced in a relatively short time interval. Due to this sudden rise in temperature, the tissue surrounding the electrode may be damaged, perhaps irreparably. Thus, nerve damage may occur because of the thermal energy dissipated into a nerve tissue coupled to the electrode of FIG. 1. State-of-the-art implantable device systems generally lack an efficient method of protecting body tissue from thermal damage due to RF or magnetic energy induced current.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.